We fly the Dash 8 with bypass doors open all the time (unless the temperature goes above ISA+25 :-) so we don't have to open and close them all the time. We also plug the up (personally) for all night stops and extended stops with precip. Nasty but seems to help.
The only theory I can think of is snowfall during turn-around. There seems to be no way the nacelle plenum chamber can ice up during engine operation. A DHC engineer once told me they threw all kinds of objects into the nacelle during testing and never managed to get anything into the engine as all solids wouldn't go with the 270 degree turn the air is forced to make.
Vastly superior to the Dash 7 which lost lots of power with the intake deflectors open...when you needed it most.
The F.50 was retrofitted with a very elaborate system of heating mats in the bottom of the air intake to avoid freezing and later "spontaneous" departure of big chunks into the engine. Also some double flameouts before the mod came out.